Iron is necessary for many fundamental cellular processes, including erythrocyte and lymphocyte development and function. Transferrin receptor 1 (TfR1), encoded by TFRC, is a widely expressed receptor critical for intracellular iron transport. Using whole genome sequencing (WGS), we have identified the first human mutation in TFRC in a novel primary immunodeficiency (PID) characterized by recurrent sinopulmonary infections, thrombocytopenia, agammaglobulinemia, absent memory B cells, defective T and B cell activation, and impaired megakaryocyte generation in vitro. Surprisingly, despite the critical role of TfR1-mediated iron transport in erythropoiesis, the patients had little or no microcytic anemia. All 14 affected members of a consanguineous Kuwaiti family were homozygous for a missense mutation in TFRC, which substitutes the tyrosine residue in the TfR1 internalization signal motif 20YTRF23 with histidine (Y20H). In contrast to resting lymphocytes from healthy individuals, which express minimal amounts of surface TfR1, resting lymphocytes from the patients express very high levels of surface TfR1. Fibroblasts from the patients also exhibit increased surface TfR1 expression and have defective transferrin uptake, indicating that the Y20H mutation impairs TfR1 endocytosis and TfR1-dependent intracellular import of iron-laden holotransferrin. TfR1 surface expression on the patients' erythroblasts was only slightly increased, suggesting that a protein expressed selectively in the erythroid lineage may compensate for the effect of the Y20H mutation on TfR1 internalization in that lineage. A potential candidate is the ferrireductase Steap3, which is selectively expressed in erythroid cells, co-localizes with TfR1, and has a 288YQRF291 internalization signal motif. We will test the hypothesis that the Y20H mutation in TFRC impairs TfR1 endocytosis, receptor-ligand internalization, and iron transport in the patients' lymphocytes. We propose that cell permeable iron chelates and/or expression of wild type (WT) TFRC correct the defects in lymphocyte function and megakaryocyte differentiation in the patients. Finally, we will test the hypothesis that Steap3 associates with TfR1, thereby promoting its internalization and iron transport, thus sparing the red cell lineage from the effects of the TfR1 mutation in the patients. Identificationof the molecular basis and pathogenesis of TFRC deficiency has important prognostic and therapeutic implications, including early diagnosis by flow cytometric analysis of cord blood lymphocytes for increased TfR1 surface expression, early intervention with hematopoietic stem cell transplantation (HSCT), and genetic counseling. The proposed studies of the mutation will be applicable to PIDs with a phenotype similar to that of TfR1 deficiency.